1. Field of the Invention
The present invention relates in general to the changing of spent anodes in aluminium production electrolysis tanks. It relates most specifically to a module which is able to allow the actual extraction of the spent anodes and to the installation employing such modules.
Aluminium is produced on an industrial scale by the now well-known process of dry electrolysis, that is to say a process involving electrolyzing alumina in a bath of molten cryolite according to the reaction: Al.sub.2 O.sub.3 (Alumina)+2e.sup.- +Na.sub.3 [AlF.sub.6 ](Cryolite).fwdarw.2 Al+3O.sub.2 +3C.fwdarw.3CO.sub.2 +F
This reaction, which is highly exothermic, therefore uses a molten bath containing a mixture of cryolite and alumina, the temperature of which is generally in excess of 800.degree. C. and consumes a vast amount of electricity, which means that the plants operate continuously, so as to limit the energy losses that are inherent in re-start phases.
The various anodes, usually made of carbon, in each of the tanks need to be replaced regularly without stopping the electrolysis reaction to do so.
Because of the process employed, namely dry electrolysis, a hard crust of fluorinated cryolite alumina forms at the upper surface of the bath, this crust having the advantage of retaining heat within the bath and thus of creating an insulating envelope.
However, extracting the spent anodes from the bath first of all requires this crust to be broken, and experience has shown that the force needed to pull a spent anode out of the said crust is seven to eight times greater than the mass of a fresh anode.
Furthermore, this force lasts for only a few centimeters of the distance, whereas the distance travelled for lifting the anode in each cycle is of the order of three to four meters, typically 3.50 m.
2. Description of the Related Art
To date, in existing plants, these operations have been performed using systems of hydraulic jacks which, to date, have only a limited size to allow them to fit into the volume available above the tanks.
However, regardless of the quality of the oil used in these hydraulic jacks, there is always a risk of fire given the high temperature of the molten bath, and there is a desire to escape from this prohibitive drawback.
Solutions hitherto proposed for avoiding the use of the hydraulic extraction system have run into a problem of size and of the civil engineering works of existing plants. Specifically, these solutions typically employ a cable system which takes up a greater amount of space which is incompatible with these plants or, requiring the production of new plants, have too significant an effect on the corresponding costs before they can be amortized.